Supercritical Fluid Science and Technology - American Chemical

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Chapter 19

Electrochemical Measurements of Corrosion of Iron Alloys in Supercritical Water 1

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Shaoping Huang , KirkDaehling ,Thomas E.Carleson ,Masud Abdel-Latif , Pat Taylor , Chien Wai , and Alan Propp 1

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University of Idaho, Moscow, ID 83843 EG&G, Idaho, Inc., Idaho Falls, ID 83415

Electrochemical potentiostat measurements have been performed for the corrosion of iron, carbon steel, and stainless steel alloys in supercritical water. The open circuit potential, the exchange or corrosion current density, and the transfer coefficients were determined for pressures and temperatures from ambient to supercritical water conditions. Corrosion current densities increased exponentially with temperature up to the critical point and then decreased with temperature above the critical point. A semi-empirical model is proposed for describing this phenomenon. Although the current density of iron exceeded that of 304 stainless steel by a factor of three at ambient conditions, the two were comparable at supercritical water conditions. The transfer coefficients did not vary with temperature and pressure while the open circuit potential relative to a silver­ -silver chloride electrode exhibited complicated behavior.

When potassium iodide was dissolved i n s u p e r c r i t i c a l ethanol and then p r e c i p i t a t e d upon reducing the pressure, the r e v e r s i b l e solvating power o f s u p e r c r i t i c a l f l u i d s (SCF) was demonstrated ( 1 ) . Since the l a s t decade, t h i s technology has been widely investigated (2,3). The electrochemical study o f metals i n s u p e r c r i t i c a l water conditions i s e s s e n t i a l f o r an understanding o f corrosion behavior. These studies are important f o r the evaluation o f b o i l e r tube f a i l u r e s i n nuclear and conventional power plants, m e t a l l u r g i c a l processing, and geochemistry o f the earth crust. There are few published electrochemical studies o f s u p e r c r i t i c a l water (temperatures and pressures above 374 °C and 218 atm). Some o f the data p e r t a i n to the power generation industry; for example, a 1975 report summarizes some I n d u s t r i a l experience with corrosion o f steam generator tubing i n pressurized water reactors (4). Most o f the other l i t e r a t u r e references concern 0097-6156/89Λ)406-0287$06.00/Ό e 1989 American Chemical Society

In Supercritical Fluid Science and Technology; Johnston, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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research r e s u l t s . A 1986 report presented, f o r temperatures up to 300 °C, the standard p o t e n t i a l of the h a l f reactions of copper(II) system i n Na2S04 and KC1 solutions, as w e l l as the d i f f u s i o n c o e f f i c i e n t of Cu(II) Q ) . W. M. Flarsheim et a l . (f>) conducted electrochemical measurements for NaHS04, KBr, KI, and hydroquinone solutions from 25 °C to 350 °C. J. B. S i l v e r , et a l . Q ) studied the corrosion and the deposition of Magnetite on two kinds of s t e e l s (2.25Cr-1.0Mo and 9.0Cr-0.1Mo) i n high temperature water under heat fluxes up to 860 kw/m . For high p u r i t y water, p o l a r i z a t i o n curves were measured at room temperature and 290 °C by M. Hishlda, et a l . (fi). They studied AISI 304 S.S., carbon s t e e l (STS 42), Ni, Cr, and Co metals. They used a dynamic IR compensated potentiostat which suppressed the e f f e c t of s o l u t i o n resistance between the working and reference electrodes. The anodic p o l a r i z a t i o n curves showed no active current peak except f o r carbon s t e e l . Other electrochemical studies conducted near the c r i t i c a l point of water include those of

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Iding (2) and Carter (Ifi). Most of the data available i n the l i t e r a t u r e are for s u b c r i t i c a l conditions. Corrosion studies o f i r o n a l l o y s i n s u p e r c r i t i c a l water have not been reported. For s u p e r c r i t i c a l f l u i d extraction and corrosion studies, a s u p e r c r i t i c a l f l u i d reactor system f o r temperatures up to 530 C and pressures up to 300 atm was constructed. This system was used to determine the electrochemical behavior of type 304 s t a i n l e s s s t e e l (304 S.S.), 316 S.S., 1080 carbon s t e e l (1080 C.S.), and pure i r o n i n s u p e r c r i t i c a l water. e

Construction of Apparatus. The schematic of the apparatus f o r s u p e r c r i t i c a l corrosion studies i s shown i n Figure 1. The important components include: a type 396-89 Simplex Minipump which can accurately meter (between 46 and 460 ml/hr) a wide v a r i e t y of solvents at pressures up to 6000 p s i (about 400 atm); an EG&G Model 362 Scanning Potentiostat; the electrochemical c e l l ; an IBM PC computer with interface hardware for electrochemical p o t e n t i a l and current, temperature, and pressure measurement and c o n t r o l ; and a 316 s t a i n l e s s s t e e l reactor, which holds the s u p e r c r i t i c a l f l u i d for the measurements. The a l l o y was selected for excellent corrosion resistance properties and r e l a t i v e l y low cost when compared with other exotic a l l o y s such as Hastelloy C. The reference electrode i s an i n t e r n a l s i l v e r - s i l v e r chloride type. I t was based on a design of A. K. Agrawal, et a l . (IX). For s u p e r c r i t i c a l water studies, the electrode body was made from 902 p r e c i s i o n machinable ceramic (Cotronics Corporation Brooklyn, NY). A type No. 29 Sauereisen low expansion cement was used for connection of the electrode body and the s i l v e r - s i l v e r chloride wire. The concentration of the KC1 s o l u t i o n i n the electrode f o r t h i s study i s 0.1 mo1/1. The i r o n a l l o y working electrode dimensions were 20mm χ 5mm χ 1.5mm. The body of the reactor served as the a u x i l i a r y electrode. Measurements. P o l a r i z a t i o n curves were obtained by remote operation of the s u p e r c r i t i c a l system by the computer and potentiostat. The e f f e c t of the IR drop i n pure water was determined by comparing the

In Supercritical Fluid Science and Technology; Johnston, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

In Supercritical Fluid Science and Technology; Johnston, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

Solvent Sourcv

FIGURE 1

APPARATUS

SCHEMATIC FOR CORROSION

STUDY

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Ooto Acquisition System Control

IBM PC/XT

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r e s u l t s obtained with a 0.005 mol/1 Na2S04 s o l u t i o n to those of pure water at the same s u p e r c r i t i c a l condition. The p o l a r i z a t i o n curves are shown i n Figure 2. The p o l a r i z a t i o n behavior i s s i m i l a r except that the current density f o r pure water i s less than that for the sodium s u l f a t e s o l u t i o n and the curve f o r pure water has a larger p o t e n t i a l range. Nevertheless, the e f f e c t of the IR drop should be quite small when the current density i s close to zero. For p o t e n t i a l s within +/- 0.2 mV of the open c i r c u i t p o t e n t i a l , the current density i s low and the two p o l a r i z a t i o n curves are close to each other. Although the potentiostat has an adjustable IR compensation c i r c u i t , t h i s was not used. When i t was used, the scan rate varied. Compensation d i d not seem to have much e f f e c t . Some researchers have developed t h e i r own IR compensation c i r c u i t f o r the high resistance s o l u t i o n (£), some added a supporting e l e c t r o l y t e (12,11) , and some simply ignored i t (H) . Since the behavior of e l e c t r o l y t e s under s u p e r c r i t i c a l conditions i s not well known, no supporting e l e c t r o l y t e was added to eliminate the IR drop i n t h i s experiment. Furthermore, the IR c i r c u i t was not used for t h i s study either, since the desired electrochemical data, such as exchange current density, open c i r c u i t p o t e n t i a l , and transfer c o e f f i c i e n t s , can be obtained from the p o l a r i z a t i o n curves without IR compensation. The p o l a r i z a t i o n curves showed that a large scan rate a f f e c t s the p o l a r i z a t i o n curves. This phenomenon can be i l l u s t r a t e d by the capacitance e f f e c t of the e l e c t r i c double layer on the working electrode surface. Based upon the model of M. Hishida, et a l . (fi), assume C